1 Field of the Invention
The present invention relates generally to illumination systems, and more particularly to an illumination system using optical feedback.
2 Description of the Related Art
Illumination systems generate light for illuminating objects including image displays. Unfortunately, the output light of conventional illumination systems often varies in intensity. The variance in intensity may be uniform across the spectral frequency of the output light or localized in a non-uniform fashion to one or more visible bandwidths (e.g. red, green, or blue light) thereof. The inconsistencies may result from a variety of factors including temperature variations of the light source of the illumination system, age related physical changes in the light source, or changes in the ambient conditions in which the light source operates.
The present invention relates to an illumination system using optical feedback to maintain a predetermined light intensity output. The illumination system employs an electrically controllable optical filter for filtering light received thereby. The illumination system also includes a light detector for detecting at least a portion of the light filtered by the electrically controllable optical filter. The light detector is in data communication with the electrically controllable optical filter. Some or all light filtered by the electrically controllable optical filter is detected by the light detector, which, in turn generates a corresponding signal that is compared to at least one predetermined value. If the signal generated by the light detector differs when compared to the at least one predetermined value, one or more filtering characteristics of the electrically controllable optical filter are varied which, in turn, varies the amount of light filtered by the electrically controllable optical filter. The filtering characteristics of the electrically controllable optical filter continue to be varied until the signal generated by the light detector substantially matches the at least one predetermined value.
In one embodiment, the electrically controllable optical filter includes one or more electrically switchable holographic optical elements. Each of the electrically switchable holographic optical elements operates between an active state and an inactive state depending upon the magnitude of a voltage received thereby. In the inactive state, light incident upon the electrically switchable holographic optical element is transmitted therethrough without substantial alteration. In the active state, the electrically switchable holographic optical element diffracts a select bandwidth of the received incident light into at least a zero order-diffracted component and a first order diffracted component. The remaining, undiffracted portions of incident light are transmitted by the activated electrically switchable holographic optical element without substantial alteration. In the active state, the intensity of light in the zero order-diffracted component and the first order diffracted component depends on the magnitude of the voltage received by the electrically switchable holographic optical element.